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Alias

Formal Definition

An alternate name for an existing named entity.

Simplified Syntax

alias alias_name : alias_type is object_name;

Description

The alias declares an alternative name for any existing object: signal, variable, constant or file. It can also be used for "non-objects": virtually everything, which was previously declared, except for labels, loop parameters, and generate parameters.

Alias does not define a new object. It is just a specific name assigned to some existing object.

Aliases are prevalently used to assign specific names to slices of vectors in order to improve readability of the specification (see example 1). When an alias denotes a slice of an object and no subtype indication is given then the subtype of the object is viewed as if it was of the subtype specified by the slice.

If the alias refers to some other object than a slice and no subtype indication is supported then the object is viewed in the same way as it was declared.

When a subtype indication is supported then the object is viewed as if it were of the subtype specified. In case of arrays, the subtype indication can be of opposite direction than the original object (example 2).

Subtype indication is allowed only for object alias declarations.

A reference to an alias is implicitly a reference to the object denoted by the alias (example 3).

If an alias denotes a subprogram (including an operator) or enumeration literal then a signature (matching the parameter and result type) is required (example 4). See signature for details.

The four aliases in the example above denote four elements of an instruction: operation code, source code, destination code and immediate data supported for some operations. Note that in all declarations the number of bits in the subtype indication and the subtype of the original object match.